Looking at the equation for cellular respiration:
glucose + oxygen → carbon dioxide + water + energy
Since cells use glucose and oxygen to make carbon dioxide, water, and energy, carbon dioxide and oxygen are the reactants, while carbon dioxide, water and energy are the products.
The purpose of cells undergoing cellular respiration is to make energy for the cells, so, energy is the main product of this reaction, while, carbon dioxide and water are the waste products, also known as byproducts.
Therefore, the answer is A, water.
